In general, the hot strip is produced by rolling a slab heated at a high temperature into a desired size, and is cooled with coolant in the hot rolling process or on the run out table after the finish rolling. The above-described cooling with the coolant is performed for the purpose of adjusting the material to obtain the intended strength and ductility by mainly controlling the deposition and transformation of the strip. The accurate control of the temperature at the end of cooling is especially essential to produce the hot strip which exhibits the intended material properties with no variation.
Meanwhile, the generally employed cooling facility (water cooling facility) for the cooling with the coolant may cause such problems as the temperature unevenness or failure to control the intended temperature at the end of cooling.
The aforementioned problems are considered to be caused by the residual coolant on the strip, which will be described taking the case for cooling the strip with the coolant on the run out table.
Generally, the upper side of the strip is cooled by vertically dropping the coolant from the round type nozzle or a slit type nozzle. When the coolant impinges against the strip, it flows forward together with the strip while being kept thereon. The residual coolant is usually discharged through purging. However, purging is performed at the position apart from the spot where the coolant impinges against the strip. The portion of the strip with the residual coolant is locally cooled to cause the temperature unevenness. Especially in the low-temperature zone at 500° C. or lower, the residual coolant in the film boiling state is transformed into the transition boiling state or the nucleate boiling state to intensify the cooling capability. As a result, the temperature difference of the strip between the portion with no residual coolant kept thereon and the portion with the residual coolant kept thereon may occur. In order to avoid the aforementioned difference, the drain purge is intensively performed. However, the transition boiling and the nucleate boiling may cause the residual coolant to adhere to the strip. It is therefore difficult to remove the residual coolant through the drain purge.
Various studies have been made to solve the aforementioned problem.
For example, Patent Document 1 discloses the structure for injecting the coolant from the slit nozzle units each provided with a lift mechanism and arranged opposite the conveying direction to stabilize the cooling operation while maintaining the cooling rate over a wide range by using the separately provided laminar nozzle and spray nozzle.
Patent Document 2 discloses the structure for injecting the film-state coolant by tilting headers each with the slit type nozzle, and filling the coolant with the space between the steel plate and a partition plate so as to establish uniform cooling at the high cooling rate.    Patent Document 1: Japanese Unexamined Patent Application Publication sho 62-260022    Patent Document 2: Japanese Unexamined Patent Application Publication sho 59-144513